TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
4 Structural, Thermal, and Spacecraft
Technology
4.1 Components Technology
4.1.1 Metrology Components
Objective
The extremely high contrast ratio requirement of the TPF coronagraph requires in turn that the
telescope secondary mirror be positioned with respect to the primary with a precision of 50 nm
over time scales of up to 24 hours. This level of position control demands precision metrology to
measure the variation in position of the secondary. The adoption of the 8 th order mask design
has significantly relaxed this requirement from the previous requirement of 300 pm with the 4 th -
order design. This activity is aimed at providing the technology necessary to measure the relative
SM-PM position.
Approach
The baseline approach is to adapt the SIM external metrology truss, based on common path
heterodyne interferometers (COPHIs), to TPF-C. The sensing scheme provides the secondary
mirror position in all 6 DOFs as shown in Figure 4-1. This involves splitting the beam into a
number of components, directing it along paths between the primary and secondary, and
interfering the outgoing and returning beams to extract the variations in distance between the two
mirrors. Key components are beam launchers, fiducials, and sources. A beam launcher is a
compact interferometer that sends a laser beam on a path whose length is being monitored.
Metrology fiducials are retro-reflectors which don’t change the path length of the beam being
reflected and must be mounted to give a stable reference for the measured path. Metrology
sources consist of two lasers, acousto-optic modulators (AOM) for frequency shifting and power
switching systems. Various associated optical components may be employed, including fiber
beam positioners, corner cube reflectors, and low loss optical cavities.
SIM metrology requirements exceed those of TPF-C in all areas with the single exception of the
frequency stability of the laser, which is required to be ~10 -9 over a 24-hour timescale.
Components SIM will develop and flight qualify include beam launchers, metrology fiducials
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